Variable Temperature Study of the Cross-Relaxation Dynamics in the Hyperpolarized Xenon-Induced Enhancement of Surface Nuclei

Abstract

The cross-relaxation rate constants for a three spin system involving hyperpolarized 129Xe, 13C, and 1H nuclei have been determined over a wide temperature range (136−216 K) through observation of the polarization enhancements from spin polarization induced nuclear Overhauser effect (SPINOE) experiments. A model system of methoxy groups adsorbed on high surface area silica was studied. A maximum polarization enhancement of 3.4 was observed for the methoxy carbon. Xenon−carbon and xenon−proton cross-relaxation were found to be dependent on both the xenon coverage and xenon diffusion across the silica surface. The temperature dependence of the cross-relaxation rate constants was effectively fit using a hopping model. Possible improvements for the SPINOE experiment are suggested including alternative substrates which would retard xenon diffusion across the surface and improve xenon−carbon cross-relaxation leading to significantly larger SPINOE enhancements.